Kuo, who wasn't involved in the new study, told Reuters Health there's been less research on when babies are developmentally ready to chew and swallow solid foods without choking - another important consideration.

"Infants should be able to sit up (and) take food off the spoon," said the CDC's Kelley Scanlon, who worked on the research.

"Sometimes if they're not ready, if they get presented with the food, they might not open their mouth... or they might spit it back up."

Her team's research included 1,334 new moms who filled out questionnaires each month about what their baby had eaten in the past week. The surveys were conducted between 2005 and 2007, when AAP recommendations called for starting solid foods no earlier than four months of age.

Just over 40 percent of parents reported their babies were eating solids, such as cereals and purees, before that point.

Those parents said they thought their babies were old enough to eat solid foods, infants seemed hungry or -- in the case of more than half of early solid feeders - doctors or nurses had recommended they start introducing those foods.

"There's not clear communication of the recommendations or the potential health impacts of early introduction," Scanlon told Reuters Health.

Nine percent of early introducers gave their baby solid food before one month, according to findings published Monday in Pediatrics.

Women who reported exclusive breastfeeding during their baby's first couple of months were less likely to introduce solid foods earlier than recommended compared to formula-feeding mothers, the CDC researchers found.

Kuo said the new findings are further evidence that pediatricians should tailor their messages about breastfeeding and solid foods to each particular parent and child -- rather than always giving "the same spiel" about introducing solids at the four-month visit.

"The decision to start solid foods in babies has to be a compromise between what makes sense for the baby and what makes sense for the mom, who most likely is working," she said.